A wind turbine usually comprises a tower and at the upper end of the tower a nacelle with a canopy which encloses the interior of the nacelle. A rotor hub of the wind turbine which comprises one or more blades is positioned at one end of the nacelle. If the generator is an indirect drive generator the rotor hub is connected to a main shaft which protrudes into the nacelle to a gearbox connected to a generator. Otherwise, if the generator is a gearless direct driven generator, the rotor of the generator is driven directly by the rotor hub where the generator is placed between and connected to the rotor hub and the nacelle. There may be direct access to the generator and the hub from the inside of the nacelle. Usually, several devices of the wind turbine are situated in the nacelle, as for example a brake system and a controller and in a geared wind turbine also the gearbox and the generator.
US 2008/0197638 A1 discloses a nacelle with a generator where a ventilator or fan is placed inside the nacelle driving outside air into the nacelle through a downwardly open gap positioned between a tower and the nacelle. The fan may together with a second fan ensure an overpressure in the nacelle and finally the air issues through an air gap on the nacelle. Moisture will not be removed from the sucked-in air. Therefore moisture will be depositing on the windings of the generator or on the installations in the nacelle reducing the life time of the wind turbine.